Let's do some math ..  Rounding examples of 7 random hypothetical purchases:
  Case Tag  
Price  
Sales  
Tax %  
Raw Total  
Incl. Sales Tax  
Rounded  
to Penny  
Rounded  
to Nickel  
Rounded  
to Dime  
  Purchase #1 $88.69   13.707%   $100.8467   $100.85   $100.85   $100.80  
  Purchase #2 $224.93   12.752%   $253.6131   $253.61   $253.60   $253.60  
  Purchase #3 $24.28   7.347%   $26.0639   $26.06   $26.05   $26.10  
  Purchase #4 $60.69   10.398%   $67.0005   $67.00   $67.00   $67.00  
  Purchase #5 $140.81   4.260%   $146.8085   $146.81   $146.80   $146.80  
  Purchase #6 $163.79   9.449%   $179.2665   $179.27   $179.25   $179.30  
  Purchase #7 $69.80   10.342%   $77.0187   $77.02   $77.00   $77.00  
  7 purchase totals = $772.99       $850.6179   $850.62   $850.55   $850.60